dm thin: clean up compiler warning

Clean up "warning: dubious: !x & y".  Also make it clear that
__snapshotted_since() returns a bool and that dm_thin_lookup_result's
'shared' member is a flag.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>